The present invention relates to fluid pressure seals and, more particularly, to bi-material elastomeric seals for sealing both low and high pressure fluids in a relatively thin annulus between a body and a rotatable shaft, such as that provided between a valve bonnet and a rotatable and axially movable valve stem.
Various types of unitary material and bi-material seals have been devised for sealing a cavity from the environment. The common elastomeric O-ring is probably the simpliest and most common form of a seal. Examples of more expensive bi-material seals are shown in U.S. Pat. Nos. 2,705,117; 3,326,560; and 4,173,129. In specialized circumstances, the need for a satisfactory seal has resulted in specially-shaped and relatively expensive seals, as shown for example in U.S. Pat. Nos. 3,052,476; 3,158,376; 4,304,415; 4,428,589; and 4,438,935.
O-ring seals are also frequently used for sealing pressurized fluid within a cavity, and a modified O-ring seal and rod wiper is disclosed in U.S. Pat. No. 3,921,991. Fluid temperatures, fluid pressures, and/or chemical or corrosive properties of the fluid often limit the use of O-ring pressure seals, however, since highly elastic sealing materials tend to experience substantially reduced seal life when subjected to these environments.
Another type of bi-material fluid pressure seal may be broadly described as an encapsulated O-ring, wherein the O-ring provides the desired low pressure seal, but an outer ring of another plastic material encapsulates the inner ring to prevent degradation of the inner more elastomeric material. These pressure seals are, however, relatively expensive to manufacture, particularly in sizes designed to seal a relatively thin annulus. Various encapsuated O-ring seals are available from Chicago Gasket Company, as shown in their Bulletin O-2 entitled "Mirror Finish TFE O-Rings."
Attempts to devise reliable and less expensive fluid pressure seals have resulted in various U-shaped or Y-shaped "lip" seals, wherein the open cavity between the lips of the seal is subjected to fluid pressure to increase the sealing force in response to increased fluid pressure. Such seals may be utilized singularly or may be stacked, as shown in U.S. Pat. Nos. 3,833,228; 4,053,163; 4,161,320; and 4,476,772. U.S. Pat. No. 3,892,418 illustrates a lip seal with an adjustable retaining ring; U.S. Pat. No. 3,563,558 illustrates a lip seal having undulating flanges to increase resiliency; and U.S. Pat. No. 3,920,254 discloses a V-shaped lip seal.
The above seals have not been widely accepted in many applications wherein the seal is subjected to both relatively low and relatively high fluid pressures, at least partially because such seals tend to experience poor sealing characteristics under low fluid pressure. Unitary material seals frequently lack long term resiliency, and low pressure leaks frequently develop after such seals are repeatedly subjected to high pressures. Other attempts to obtain a reliable low pressure and high pressure seal have resulted in dual-material U-shaped or Y-shaped pressure seals, wherein fluid pressure is utilized to provide the primary sealing mechanism under high fluid pressure, and the resiliency of a second plastic material is intended to provide a low pressure seal. Examples of bi-material lip seals are shown in U.S. Pat. Nos. 3,653,672; 3,885,801; 4,013,299; 4,174,846; 4,193,606; and 4,328,972. These seals typically have a relatively massive base, with the base width being slightly less than the annulus spacing. Further examples of single material and bi-material seals are disclosed in the brochure entitled "Parker Seals--Total Hydraulic/Pneumatic Sealing Systems", PPD3700, distributed by The Parker Hannifin Corporation.
Although the above-described uni-material bi-material lip seals have found acceptance in many applications, such seals have at least three significant drawbacks which limit their acceptance in many other applications: (1) many of such seals are relatively expensive to manufacture, wherein the configuration of both the interior and exterior surfaces of the sealing member require special machining or forming techniques; (2) such seals typically cannot be easily "downsized" to fit within thin annular cavities, partially because a very small material diameter would be required for the inner O-ring; and (3) the inner O-ring material, being subjected to the pressurized fluid environment, would frequently experience a relatively short life and result in the loss of a low pressure seal.
The disadvantages of the prior art are overcome by the present invention, and an improved bi-material pressure seal is hereinafter provided. In a typical application, the seal of the present invention may be used in a relatively thin annulus between a valve bonnet and a rotatable valve stem. Also enclosed are novel techniques for forming such a seal and positioning the seal within the desired annulus.